[Routing] Leave motorway, turn onto track

[Jon Bright]

Hi Frederik,

Frederik Ramm wrote:
> 
> I wonder if it would nevertheless make sense for routing engines to  
> implicitly disallow leaving a motorway on anything else than a  
> motorway_link (or at least not on something as small as a track).

You'd certainly want to allow leaving a motorway onto trunk, primary and 
secondary.  Near where I live, the A46 (motorway) ends and turns into 
the B326 (primary):

http://www.informationfreeway.org/?lat=51.31285458075313&lon=7.27426070665261&zoom=17&layers=B0000F000F

Something similar happens east of London, where the M25 (motorway) 
becomes the A282 (trunk) for the Dartford river crossing:

http://www.informationfreeway.org/?lat=51.48429693501172&lon=0.2717440463221352&zoom=15&layers=B0000F000F

An example with secondary would be this bit of the A44:

http://www.informationfreeway.org/?lat=51.32945959272296&lon=7.016253712974789&zoom=17&layers=B0000F000F

My suggestion for a solution would be to explicitly exclude "track".  In 
theory, as you say, even this shouldn't be necessary.  I don't think you 
even need turn restrictions - as far as I'm aware, pretty much every 
such track has access restrictions preventing its use as an exit.  But 
in practice, not everyone will have tagged everything perfectly, so 
excluding "track" (and maybe also "unclassified") seems like a 
reasonable compromise.

(A more complicated and computationally intensive way of doing this 
would be to disallow turns where the turn angle is greater than 45 
degrees from the motorway and the motorway itself continues.  But I 
think the better way to deal with that would be to analyse the OSM data 
for such cases, produce a list and have "gardeners" enter appropriate 
access or turn restrictions.)

Another case you might want to think about while looking at this is 
places like:

http://www.informationfreeway.org/?lat=51.94166322328894&lon=7.553746825469445&zoom=15&layers=B0000F000F

This is a normal rest stop on the motorway (between Münster-Süd and 
Münster-Nord.  The quickest way to reach some parts of Münster is to 
leave the motorway at the rest stop, take the road leading off parallel 
to the motorway and then join the K5.  But it's also illegal to do so.

Refusing ever to turn from a motorway onto highway=service might 
therefore also be appropriate.

Jon